1. Field of the Invention
This invention relates to a variable valve timing system for an internal combustion engine. More particularly, this invention relates to a torque pulse actuated, hydraulic variable valve timing system of the foregoing type with locking capabilities to lock the components of the system in a fixed condition of operation during intervals of low hydraulic pressure, such as during engine start-up.
2. Description of the Prior Art
U.S. Pat. No. 5,107,804 (Becker et al.), which is assigned to the assignee of this application, the disclosure of which is incorporated by reference herein, describes a vane-type, camshaft torque pulse actuated hydraulic camshaft or valve timing system for an internal combustion engine in which the hydraulic fluid that operates the camshaft phase shifting system is engine oil. Such a system has many operating advantages over other known types of valve or camshaft timing systems, for example, in the timeliness of response to changes in engine operating conditions. However, such systems tend to be noisy or otherwise unstable during periods of low engine oil pressure, which can often occur during engine start-up and can occasionally occur during other types of operating conditions. During these times it is important to be able to lock the otherwise relatively movable components of the system into fixed positions relative to one another, and it is to the provision of an improved solution of the system locking requirements of such a variable valve timing system that the present invention is directed and of suitable systems for controlling the operation of such a system.
U.S. Pat. No. 2,861,557 (Stolte) also describes an hydraulic variable camshaft timing system, albeit a system that is operated solely by engine oil pressure. This reference teaches that it is desirable to lock the otherwise variable components of the system in fixed positions relative to one another during low speed operation conditions, but only teaches a system in which only a single set of fixed positions can be achieved.
A variable valve timing system, of which a variable camshaft timing system is a recognized type, according to the present invention preferably is, like the system of the aforesaid ""804 patent, a camshaft torque pulse actuated, engine oil powered hydraulic system that is used to change the position of a lobed vane, albeit a vane with three lobes rather than the two-lobed vane of the ""804 patent, within lobe receiving recesses of a surrounding housing. According to the present invention, however, the vane and the housing are locked in fixed positions relative to one another by a locking plate that is spring biased, against the effects of engine oil pressure, to prevent relative motion between the vane and the housing except when the engine oil pressure exceeds a predetermined value, and the locking can occur at one or another of a multitude of positions of the vane and the housing relative to one another. It is also contemplated that the invention can be adapted to a hybrid variable camshaft timing system operated both on engine oil. pressure, and oil pressure resulting from camshaft torque pulses, such as that of U.S. Pat. No. 5,657,725 (Butterfield et al.), which is also assigned to the assignee of this application, the disclosure of which is also incorporated by reference herein, and to an engine oil pressure actuated system such as that of the aforesaid U.S. Pat. No. 2,861,557.
A camshaft torque pulse actuated hydraulic VCT system, or a hybrid system that operates both on engine oil pressure and oil pressure generated by camshaft torque pulses, can be locked in place by the locking arrangement of the present invention, which lends itself to on-off control in various ways, depending on the needs or wishes of the user. First, a solenoid can be employed to control the application of engine oil pressure against the locking place to prevent unlocking of the vane and the housing unless and until the solenoid is de-energized, even when engine oil pressure exceeds the predetermined value. This will permit the relative positions of the vane and the housing to be changed from a given locked position to a different locked position even when the engine oil pressure exceeds the predetermined value. Alternatively, the engine oil pressure can be applied directly against the locking plate, without any attempt to selectively isolate the locking plate from the effects of engine oil pressure, so that the engine timing system will always be operable during periods of high engine oil pressure.
The variable valve timing/variable camshaft timing system of the present invention can also be controlled during operation either by an open loop system or a closed loop system, again depending on the needs or wishes of the user. In an open loop control system, there are only two control positions, either a position where the vane moves at a fixed rate to full advance or a position where the vane moves at the fixed rate to full retard, without any effort to modulate the rate of movement of the vane to its full advance or full retard position, as the case may be, or to stop the movement of the vane at any position in between such full advance and fill retard positions. In a closed loop control system, on the other hand, the position of the vane relative to the housing is monitored and the system is locked at one or another of a multitude of possible relative positions of the vane and the housing between the full advance and full retard positions.
Accordingly, it is an object of the present invention to provide an improved vane-type, torque pulse actuated, hydraulic variable valve timing, or variable camshaft timing system for an internal combustion engine. More particularly, it is an object of the present invention to provide a variable valve timing or variable camshaft timing system of the foregoing character with an improved arrangement for locking a position of a vane relative to a position of a housing in which the vane is normally free to move, whenever engine operating conditions make it desirable to prevent relative motion between the vane and the housing.
It is also an object of the present invention to provide improved control systems for controlling the operating of a variable valve timing or variable camshaft timing system of the foregoing character.
For a further understanding of the present invention and the objects thereof, attention is directed to the drawing and the following brief description thereof, to the detailed description of the preferred embodiment and to the appended claims.